DISPERSION OF MINE-TAILINGS DERIVED MERCURY THROUGH GROUNDWATER-SURFACE WATER FLOW SYSTEM

 

Gossan Creek, a first-order stream in the Upsalquitch River watershed in northern New Brunswick contains elevated concentrations of Hg and other heavy metals, as a result of leaching from a gold-mine tailings disposal site. Field and laboratory research reveals that the tailings pile acts as a point source of mercury into the aqueous system. A contaminated groundwater plume originates from the tailings pile and discharges into the headwaters of Gossan Creek. The groundwater and the headwaters of the stream system contain high concentrations of Hg (up to 60 ug/L), Cu, Al, Fe, Zn, Mn.Ni, Co, As and cyanide (CN). The concentrations of Hg and CN display a strong positive correlation (r²=0.911) in the ground water plume and the creek water, suggesting that Hg has been mobilized from the tailings in the form of highly soluble Hg-CN complexes.

 

The mass-flux of Hg_T (total of all Hg species) has been measured in the stream and it is observed that 95-99% of the flux is attenuated within 4 km from the source. The loss of Hg from the water can be attributed to accumulation in the sediments and biota, and to evasion from the stream to the atmosphere.

 

The evasion of Hg⁰ from the creek water to the atmosphere has been investigated in detail. Existing methods for measurement of Hg⁰ evasion flux from water bodies cannot be applied with satisfactory results in the case of first-order streams due to their complex morphology and highly turbulent flow. We therefore attempted a new approach, involving field tracer test and laboratory gas-indexing experiments to measure the water/atmosphere gas exchange coefficient for Hg⁰ under field conditions.  The exchange coefficient has been used to estimate the potential Hg⁰ evasion flux from the creek. The observed rate of decrease in Hg_T mass-flux in the creek is less than the maximum potential evasion flux estimated on the basis of experimentally determined Hg⁰ exchange coefficients.  This suggests that Hg⁰ evasion is not the rate-determining mechanism for the loss of Hg from the stream. There are a number of processes that could be responsible for limiting the overall evasion rate from the stream, however the observed strong correlation between the attenuation of both Hg_T and CN in the creek suggests that the rate of Hg-CN complex dissociation may be the limiting process.